1. Field of the Invention
The present invention is related to a method for the manufacturing of a moulded body on basis of a raw material which is the material left after remoulding pulp from paper and extraction of fibres for manufacturing of paper, which rawmaterial in principle contains water, short cellulose fibres, and fine, mineral particles.
The invention concerns additionally a moulded body produced by this procedure and a plant for exercising the method.
2. Background Art
From GB patent specification No. 1 275 042 is known a method for utilizing papermud for producing a board-shaped product or a mouldable mixture. The raw material used is papermud, i.e. an aqueous suspension which passes through the filter belt when cellulose pulp is manufactured in a fourdrinier apparatus for the manufacturing of paper. To this raw material which contains short fibres and inorganic minerals is added fibres of middle-length within the interval 1/16"-5" (1.6-127 mm), and it is concentrated by passing a rotating, perforated roller, and is layed out in a filter belt press which produces an endless fibrous length. This material can after pressing and drying be used as filling material in board-shaped products. Boards of higher strength may be obtained by adding cement, gypsum or adhesives to a liquid mixture.
From GB patent specification No. 1 405 587 is known a development of this technique, where the aquous suspension is layed out on a support permeable to liquid, in the form of an endless filter belt and pressed through a set of rollers with gradually narrower gaps to force out the water. Hereby is produced a layer having some wet strength, which can be formed under pressure and heat. It is noted that the mould may be perforated so as to let water escape while the body is kept under pressure and heat, and it is noted as a second possibility that in between the surface of the mould and that of the body may be inserted a flexible tissue which may be founded on to the surface of the finished body. Adhesive substances are added to the raw material as required. The heat founding can for example be carried out at 160.degree. C. at a pressure of 37 kp/cm.sup.2 and a residence time of 15 minutes. The finished product has a moisture content of approx. 15%.
In GB patent specification No. 1 416 321 the same inventors describe a similar process and in that patent explain that the flexible tissue which can be inserted in either one or both sides of the body during pressing has the function of keeping the material together in the mould and to make it easier for liquid or steam escape from the body.
From Swedish patent specification No. 85 128 it is known to produce boards from paper pulp according to a method comprising shaping, pressing, drying and, finally, hot-pressing carried out at gradually increasing pressure. It is explained that the last pressure step is initiated when the boards actually begin to warp. The specification also states the fact that vegetable fibres contain water, partly as moisture in the holes of the wooden cells and partly absorbed by the fibres and stays there also after air-drying. It is also noted that in the hot press it is suitable to place a wire netting on one or both sides of the board for easier steam exit. This, of course, causes hobbing of a corresponding pattern into the board surfaces. If the wire netting is not used, the moisture content to be removed during the hot pressing, can practically escape only via the border slit, which results in a very long pressing time, and the process will be unrealistic except for the production very small board sizes.
Within the woodworking industry it is known to produce particle boards by producing a liquid slurry from crushed wooden fibres, water and glue in a filter curtained press and to later on harden the particle boards in a hot press. The required pressure is very high and a considerable quantity of glue is used to achieve a continuous unbroken board with a suitable density. Steam explosion of the boards is a very important factor which has to be overcome by keeping a high pressure for a comparatively long period of time, during which the boards are allowed to dry and the glue is hardened. As hot press is a rather high initial investment, and as the press is occupied for a quite long time per board, depending on the thickness of the board, the pressing operation will represent an important part of the production cost.
From U.S. Pat. No. 1,578,609 it is known to produce thin wood fibre boards according to a process where wood chips are crushed and softened under heat treatment and subsequently pressed. According to that professional process, the softened chips are pressed at very high temperatures in a press in which a filter net is inserted on one side so as to allow steam and water to escape during the pressing. These boards, which are known under the commercial name Masonite, are produced without glue but kept together only by the natural content of lignin from the wood chips. The filter curtain causes the hobbing of a pattern on one side of the manufactured boards.
It is known to produce Masonite boards which are smooth on both sides. These are produced after the above mentioned method but before the boards are completely dried the boards are pressed in a press with smooth pressplanes. The press is further equipped with heated planes and the temperature is preset to a temperature where the lignin in the Masonite board melts and a Masonite board with smooth surface is produced.